patch 9.1.0016: default diff highlighting is too noisy

Problem:  default diff highlighting is too noisy
Solution: Link diff highlighting groups to new
          Added/Removed/Changed, revert previous change
          (Romain Lafourcade)

Remove diff* links added in #13776 and doc added in commit b1392be

The links added in #13776 are way too noisy for the contexts in which
the `diff` syntax is applied (git commits, patches, etc.).

This commit:

- removes those links
- adds new default highlighting groups Added, Changed and
  Removed
- links the diff highlighting groups to those new defaults
- removes the doc changes
- adjusts the syntax_completion test for those newly added group
  names

Note: Changes to the default color schemes will be handled separately,
by adding links to those newly created Added/Removed/Changed
highlighting groups.

related: #13776
closes #13825

Signed-off-by: Romain Lafourcade <romain.lafourcade@razorfish.fr>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/syntax.txt

@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2024 Jan 03
+*syntax.txt*	For Vim version 9.1.  Last change: 2024 Jan 06
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -271,6 +271,10 @@ you can see the actual color, except for "Ignore"):
 	*Todo		anything that needs extra attention; mostly the
 			keywords TODO FIXME and XXX
 
+	*Added		added line in a diff
+	*Changed	changed line in a diff
+	*Removed	removed line in a diff
+
 The names marked with * are the preferred groups; the others are minor groups.
 For the preferred groups, the "syntax.vim" file contains default highlighting.
 The minor groups are linked to the preferred groups, so they get the same
@@ -1261,7 +1265,7 @@ To highlight KDE-reserved features, set >
 g:desktop_enable_kde follows g:desktop_enable_nonstd if not supplied
 
 
-DIFF						*diff.vim* *ft-diff-syntax*
+DIFF							*diff.vim*
 
 The diff highlighting normally finds translated headers.  This can be slow if
 there are very long lines in the file.  To disable translations: >
@@ -1270,15 +1274,6 @@ there are very long lines in the file.  To disable translations: >
 
 Also see |diff-slow|.
 
-Since the Vim 9.1 release |version-9.1| the diff filetype links the diffAdded,
-diffRemoved and diffChanged highlighting groups to |hl-DiffAdd|,
-|hl-DiffDelete| and |hl-DiffChange| by default.  If you do not want this, you
-can change it to the previous groups like this in your |.vimrc| >
-
-	hi link diffRemoved Special
-	hi link diffChanged PreProc
-	hi link diffAdded Identifier
-<
 DIRCOLORS			       *dircolors.vim* *ft-dircolors-syntax*
 
 The dircolors utility highlighting definition has one option.  It exists to